A permanent-magnet synchronous motor (hereinafter, simply “motor”, except when it is necessary to distinguish the permanent-magnet synchronous motor from other motors) is different from an induction motor that is commonly used in the following points:
(1) Because magnetic flux is established by a permanent magnet incorporated in a rotor, an exciting current is unnecessary.
(2) Because no current flows through a rotor unlike the induction motor, no secondary copper loss is generated.
(3) Because the permanent-magnet synchronous motor has the characteristics (1) and (2), this motor is known as an efficient motor.
Although the induction motors have been commonly used in electric vehicles, use of the permanent-magnet synchronous motors that have the above mentioned better characteristics is recently considered to further improve the efficiency.
Generally, in an electric vehicle, generally a plurality of cars connected to one another. Moreover, a plurality of power converters and motors are decentrally mounted on the plural cars. Therefore, even if some of the power converters of the electric vehicle fail and some of the motors fall into an inoperative state, the electric vehicle can keep running by the remaining working power converters and motors.
In the permanent-magnet synchronous motor, unlike in the induction motor that is commonly used, even if power is not supplied from outside, that is, even if the power converter comes to a stop, an induced voltage is inevitably generated at a terminal of the motor during rotation of the rotor by an action of magnetic flux of the permanent magnet incorporated in the rotor of the motor. As a result, even if the power converter fails and stops operating while the electric vehicle is running, the rotor of the motor keeps on rotating due to the rotation of the wheel that is directly connected to the rotor while the electric vehicle is running. Accordingly, the motor keeps generating an induced voltage that is proportional to the number of revolutions of the motor.
Because persons skilled in the art know this event well, the power converter is designed to withstand the induced voltage.
A short-circuit fault can occur in the power converter. When such short-circuit fault occurs, a circuit path that short-circuits the induced voltage from the motor may be constituted. That is, a short-circuit current can flow in the motor due to the induced voltage.
Practically, the present inventors have confirmed that when terminals of a motor designed for a certain electric vehicle are short-circuited under a condition of the maximum number of revolutions, a short-circuit current up to several hundreds amperes could be generated.
In the case of a system that is driven by one motor such as an electric car, when the short-circuit fault described above occurs in the power converter, the vehicle immediately falls into a non-operating state, and rotation of the motor is stopped. As a result, the short-circuit current quickly decays.
In the electric vehicle to which the present invention is applied, however, even when the short-circuit fault occurs in some of the power converters during running, the electric vehicle keeps running by the remaining operating power converters and motors. As a result, the short-circuit current due to the induced voltage of the motor keeps flowing through the faulty portion (short-circuit location) of the power converter where the short-circuit fault has occurred. If this state is left unsettled, there is a possibility that the damage of the faulty portion of the power converter becomes worse or the faulty portion or the motor becomes hot or in worst cases burns, and this can cause a serious problem.
Naturally, in the case of the conventional induction motor, no short-circuit current flows even if terminals of the induction motor during rotation are short circuited. Therefore, the problem of this kind does not exist in the conventional induction motor.
To solve this problem, Patent Document 1 mentioned below teaches providing a contactor. The contactor is an opening/closing unit for electrically isolating a connection between an inverter as a power converter that drives and controls a permanent-magnet synchronous motor and the motor. Accordingly, the damage of the inverter does not become worse by an induced voltage of the motor when the inverter breaks down while an electric vehicle is running. Moreover, when a controller detects a trouble of the inverter, the controller turns the contactor OFF to isolate the inverter and the motor from each other.    Patent Document 1: Japanese Patent Application Laid-open No. H8-182105